Process for coating paper by moulding

ABSTRACT

THIS INVENTION RELATES TO A PROCESS FOR TREATING PAPER AND OTHER FLEXIBLE SUPPORTS SO AS TO PRODUCE A COATED PRODUCT WHICH PROVIDES A MORE UNIFORM COATING AND A GLOSS GREATER THAT CAN BE OBTAINED BY CONVENTIONAL PROCESSES FOR MAKING STUCCO PAPER.

Patented Aug. 29, 1972 Int. (:1. B281) 7738; D21h N48 US. Cl. 117-64 C 1 Claim ABSTRACT OF THE DISCLOSURE This invention relates to a process for treating paper and other flexible supports so as to produce a coated product which provides a more uniform coating and a gloss greater that can be obtained by conventional processes for making stucco paper.

This invention relates to a process for treating paper and other flexible supports or substrates so as to produce in a single working step a coated product which can be printed by any known processes such as offset, letterpress, intaglio, etc. The product also has a very smooth surface and a gloss greater than can be obtained by conventional processes for making stucco paper; also the conventional processes do not provide the same uniformity of coating as does this invention.

In conventional coating methods, the high pressures under which the paper is treated in the finishing operation make it impossible for the coating surface to be completely regular.

The present process resides in applying to one side or surface the substrate by some conventional means (air blade, rollers, etc.) an aqueous suspension basically comprising one or more mineral pigments, such as kaolin, calcium carbide, satin white and titanium dioxide, and an adhesive such as casein or starch or latex, and While still Wet-i.e., mouldablepressing the coated surface lightly against a heated polished metal surface, such as a cylinder or endless belt, and then allowing it to dry, to produce in the coated surface an exact mirrorimage copy of the polished metal surface on which it dries. It is left in contact with the polished surface long enough to be removed therefrom without tearing.

The function of the adhesive is to bind the pigment to the substrate and lessen the tendency of the coating to stick to the polished surface during the finishing step. Separation agents can cooperate to the same purpose.

A synthetic adhesive having elastomeric and thermoplastic properties can be a single resin or a combination of resins; in the latter case, a hard plastic or plastics is usually combined with an elastomer to provide the required flexibility.

An aim of this invention is to produce a coating composition basically being an aqueous suspension of mineral pigments (kaolin, calcium carbonate, satin white and titanium dioxide) andan adhesive (starch, casein, latex, etc.) from which most of the air introduced during preparation has been removed. The disadvantages of having air bubbles in a coating composition appear in the quality of the stuccoed surface and to some extent govern the speed of the stuccoing operation.

Another aim of this invention is to provide a method to increase stuccoing operation efi'iciency, more particularly during the phase of drying the water contained in the coating composition.

Another aim of this invention is to produce, in coating compositions comprising mineral pigments and adhesives, a state of not sticking to a chromium-plated and polished metal surface.

Another aim of this invention is to provide a stucco paper which when out into sheets presents as a flat surfacei.e., it has no tendency to cockling or corrugate in a manner making it difficult to handle subsequently.

Another aim of this invention is to provide a coated paper whose specific volume is greater and more uniform than can be achieved by conventional processes.

The method for performing this invention can be described as follows:

A strip of paper or flexible material from a reel goes to a conventional section in which one of its sides has a coating composition applied to it, whereafter, and while the coated paper surface is still wet-i.e., mou1dableit is pressed by a rubber-clothed roller against a polished heated metal surface, such as a cylinder or endless belt; as such surface rotates, the water contained in the coating evaporates and the paper. when sufliciently dry, does not stick to the metal surface, and when the paper spontaneously disengages from the surface it contains a mirror image thereof.

To produce a flawless surface on a paper various requirements must be met.

First, most of the air which is bound to be introduced into the coating composition during the preparation thereof-i.e., during pigment dispersion, dissolution of adhesives and screening-must be removed. The presence of air bubbles in stucco paper leads to the appearance of spots of different sizes which are substantially devoid of coating and make the paper useless. To obviate this disadvantage, the coating composition in the tube supplying the applicator section goes through a deaerator of the kind used in the cosmetic paste and cream and foodstuffs industries; a low-speed, low-centrifuged-acceleration and moderate-vacuum device provides from to elimination of the occluded air in the coating composition. This step automatically obviates the associated defects in the stuccoed surface of the paper, since the coating composition used is substantially bubble-free in the manner essential for the process.

A flaw can be produced in the glossy stuccoed surface by any solid particle vehicled along on the uncoated side of the paper or coming from anywhere else in the installation, such as coating composition which has dried on the edges of the metal surface not covered by the paper and which has been deposited on the rubber-clothed roller pressing the paper against the metal surface at the first place of contact. Since it is substantially impossible to prevent such impurities from entering this section, an extra facility which continuously cleans the pressing roller has been used satisfactorily.

As already stated, once the coating is dry enough, the paper ceases to stick to the metal surface and disengages on its own, so that a mirror image of the metal surface is obtained on the stuccoed surface of the paper. However, it is found that when the metal surface is a chromiumplated polished material, the papers manufactured by this process have the disadvantage that when the coating has dried, the coated paper does not completely disengage from the metal surface, some particles of coating being attracted by and sticking tightly to the chromium surface, with detriment to the quality of the paper due to the resulting surface defects. Various solutions of the problem were studied but none was fully satisfactory. For satifactory operation a non-sticking state must be produced in the chromium-plated metal surface; the chromium can exist in an active state--i.e., chemically active and in a neutral state, and it has been found that to ensure that coating compositions including aqueous suspensions of mineral pigments and adhesives do not stick to a chromium-plated metal surface, the same must be in this neutral state--i.e., a state of inertia and inactivity in respect of chemical agents. According to this invention, to produce a neutral state of the chromium, a periodic polishing operation can be given using felt discs to which a paste consisting of a mixture of chromium oxide and fatty substances such as disclosed in Pat. 3,014,833, has been applied. Although the process is completely unknown, the result is very satisfactory, and it has been found possible to maintain a non-sticking condition for periods of from 40 to 50 hours after each such polishing step.

One of the main difficulties with this form of stuccoing is the low output compared with other methods. Rates of output may vary from 15 to 40 metres/min; endeavours to improve output by increasing the temperature of the metal surface and therefore the drying capacity leads to a reduced tion of quality (less gloss) and the frequent appearance of irregularities in the coated surface, due to the dispersion agent of the coating boiling. The improvement according to this invention is to eliminate the vapour buffer arising from evaporation of the water contained in the coating and in the paper moisture content, such vapour accompanying the paper strip during the passage thereof around the metal surface; also, in the zone near the paper there is a vapour-saturated atmosphere which reduces the evaporation rate and impairs drying. To overcome this disadvantage, a stream of hot air is directed on the uncoated side of the paper across the whole width thereof; the stream vehicles away the water vapour accompanying the strip and thus helps to speed up the drying of the coating; output can be improved by from 15 to 25%, according to paper and the applied weight of coating. v

It is familiar that papers coated on just one side tend to cockle and corrugate, more particularly if they are out into pieces, and since most of the subsequent processing machinery requires the material for treatment to be substantially fiat, this disadvantage leads to problems which react on the quality of printing and also on the efliciency of handling operations.

In the case of conventional stucco paper, this tendency is reduced during finishing operations, more particularly during calendering, which from this point of view, as it were, irons or flattens the paper. n the other hand, this opportunity does not occur in coating processes based on shaping against a metal surface, since the coated paper is given substantially complete finishing in this step. When a paper strip goes, with pressure, between a rigid member (the metal-surfaced cylinder) and a flexible member (the rubber-clothed roller), the same undergoes a deformation causing the paper to lengthen and modifying the dimensions thereof-i.e., the rigid member sinks slightly in the rubber-clothed roller so that the contact between the rigid member and the flexible member is not a line of negligible width but a zone of a width depending on the applied pressure, the hardness of the flexible roller and the diameter thereof. The end result is that the coated paper tends to cookie and not stay flat; this disadvantage depends upon a variety of factors, e.g. moisture in the paper, its weight, working pressure in the contact zone and hardness of the rubber-coated flexible roller. Studies and experiments have been made in which these factors have been varied, but without any fully satisfactory result being achieved, the paper staying flat in the best case within very narrow environmental limits. According to the improvement provided by this invention, the uncoated side of the paper is treated with a compensating layer in an amount and of a nature depending upon paper weight, the compensating layer basically comprising a solution of natural adhesive mixed with hygroscopic substances which facilitate conditioning of the paper. The treatment is given in a special filter press in a way so that the glossy coated surface does not lose any of its properties; such press is disposed in the same machine between the drying section and the rolling section.

In the manufacture of papers with mineral coatings, the manufacturing process also comprises further subsequent steps usually performed outside the coating machine;

very often, the paper goes through glazing or dofrering calenders to give it the required surface finish for presentation and as technically necessary for subsequent use.

Consequently, and because of the high working pressures used, the specific volume is reduced considerably and there is an increased risk of irregularities being formed across and along the paper strip. In the present process, the stucco paper is given its final state in the same coating machine and requires no further finishing step; also, since the water contained in the mineral coating is evaporated through the base paper, the fibres thereof swell and help to increase the specific volume. The difference from papers stuccoed by other processes can vary from '15 to 25%, an increase which, taken together with the improved regularity, for the reasons hereinbefore given, represents a considerable advantage in the operations which users of such papers need to perform.

I claim as my invention:

1. A process for mould coating a flexible substrate, such as paper, with a pigment and binding composition comprising:

(a) removing substantially all of the air from the composition to render such composition substantially free from air bubbles;

(b) applying such air bubble-free composition to one side of the substrate;

(0) pressing the coated side of the substrate while still wet with the coating against a polished heated chromium surface;

(d) maintaining such chromium surface in a neutral state of inactivity with respect to the components of the composition by periodically polishing the same with a paste consisting of a mixture of chromium oxide and fatty substances so that components of the composition do not adhere thereto and whereby for such reason the substrate can separate from the surface in its entirety and without tearing;

(e) heating the surface to a sufficiently high temperature as to dry the coating and provide for the self-dis engaging release of the substrate from the surface with the dry coated side of the substrate containing a mirror image of the surface;

(f) directing a stream of hot air on the uncoated side of the substrate to remove the vapor barrier arising from the evaporation of the moisture content in the composition as the substrate is drying on the surface and thereby to aid in the drying of the substrate on the surface; and

(g) treating the uncoated side of the substrate with a compensating layer of a hygroscopic mixture under a pressure insuificient to effect the coated side so as to cause the coated side to lose any of its properties but sufiicient to condition the substrate so that it is smooth with the coated side having a glossy quality.

References Cited UNITED STATES PATENTS 2,997,406 8/1961 Freeman etal 111 -64 2,360,825 10/1944 Blickensderfer 1l7--64 2,920,698 1/1960 Hornbostel 117-64 3,014,833 12/1961 Lee 117-64 FOREIGN PATENTS 481,025 2/1952 Canada 1l7Anti-Stick Dig.

816,679 1/1959 Great Britain 11764 WILLIAM D. MARTIN, Primary Examiner W. R. TRENOR, Assistant Examiner US. Cl. X.R. 

